Runtime error - called id for nil in Rails 3 project - ruby-on-rails

Question

I'm getting this error when I try to submit my form (/POSTS/SHOW):
RuntimeError in Posts#show
Showing /Users/fkhalid2008/loand/app/views/posts/show.html.erb where line #1 raised:
Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id
Extracted source (around line #1):
1: <%= form_remote_tag (:update => 'message', :url => {:controller => 'main', :action => 'send_message', :user_id => #post.user.id}) do %>
2: <br>
3: <br />
4: <br />
How do I fix this?
Relevant code is below:
/VIEWS/POSTS/SHOW
<%= form_remote_tag (:update => 'message', :url => {:controller => 'main', :action => 'send_message', :user_id => #post.user.id}) do %>
<br>
<br />
<br />
<div class="field">
Hello! My name is <%= f.text_field :subject %> and I'm contacting you in response to your ad. I'm interested in learning more so get in touch! Here's my contact details: <%= f.text_field :body %>.
Submit
<% end %>
POST MODEL
class Post < ActiveRecord::Base
belongs_to :user
attr_accessible :title, :job, :location, :salary
validates :title, :job, :location, :salary, :presence => true
validates :salary, :numericality => {:greater_than_or_equal_to => 1}
default_scope :order => 'posts.created_at DESC'
end
USER MODEL
class User < ActiveRecord::Base
has_many :posts
has_one :profile
has_private_messages
attr_accessible :email
validates_presence_of :email
validates_uniqueness_of :email, :message =>"Hmm, that email's already taken"
validates_format_of :email, :with => /^([^\s]+)((?:[-a-z0-9]\.)[a-z]{2,})$/i, :message => "Hi! Please use a valid email"
end
POSTS CONTROLLER
def show
#post = Post.find(params[:id])
respond_to do |format|
format.html # show.html.erb
format.json { render :json => #post }
end
end
def new
#post = Post.new
#post.user = current_user
respond_to do |format|
format.html # new.html.erb
format.json { render :json => #post }
end
end
def edit
#post = Post.find(params[:id])
end
def create
#post = Post.new(params[:post])
#post.user = current_user
respond_to do |format|
if verify_recaptcha && #post.save
format.html { redirect_to :action=> "index"}
format.json { render :json => #post, :status => :created, :location => #post }
else
format.html { render :action => "new" }
format.json { render :json => #post.errors, :status => :unprocessable_entity }
end
end
end
def update
#post = Post.find(params[:id])
#post.user = current_user
respond_to do |format|
if #post.update_attributes(params[:post])
format.html { redirect_to #post, :notice => 'Post was successfully updated.' }
format.json { head :ok }
else
format.html { render :action => "edit" }
format.json { render :json => #post.errors, :status => :unprocessable_entity }
end
end
end
APPLICATION CONTROLLER (this is where I am defining current_user)
class ApplicationController < ActionController::Base
protect_from_forgery
private
def current_user
#_current_user ||= session[:current_user_id] &&
User.find_by_id(session[:current_user_id])
end
end
MAIN CONTROLLER (send_message is defined here)
class MainController < ApplicationController
def send_message
message = Message.new
message.subject = params[:subject]
message.body = params[:message]
message.sender = User.find session[:user]
message.recipient = User.find params[:user_id]
if message.save
ContactMailer.deliver_message_email message.recipient.email, message.id, request.host
return redirect_to "/posts"
else
render :text => "Hmm. Something seems to be wrong...let me look into it"
end
end

Answer 1

You don't have a user assigned to the post record represented by the #post instance variable.
Presumably a user needs to be logged in to make a post?
Also presumably you have a current user defined somewhere?
Your controller actions that use this form need to assign the user to the post record
def new
#post = Post.new
#post.user = current_user # You will need to get the current user from somewhere
respond_to do |format|
format.html # new.html.erb
format.json { render :json => #post }
end
end
UPDATE
To make sure that your current user is assigned you should add a check to ensure the user is logged in in the controller actions. This is normally done by adding a before filter to authorize the current user which will redirect back to the login page if the current use is logged out.
Have a look at this rails cast to explain logging in and out and redirecting on a before filter http://railscasts.com/episodes/250-authentication-from-scratch
There is a revised version of the cast here but you will need a subscription for that
http://railscasts.com/episodes/250-authentication-from-scratch-revised
well worth paying for IMO
End of update
You will need to / should also assign the current user in whatever actions update the post record - i.e. the create and update actions in EXACTLY the same way.
Also, because you have not got a user assigned to a post record then you need to handle this scenario in the form so that you don't get 500 errors
You can use the #post.user.blank? boolean check to help you with this
Something like
<% if #post.user.blank? %>
<h2>There is no user assigned to this post record! This should never happen ad you should never see this message, please contact support if etc... </h2>
<% else %>
<!-- Place all your current form code here -->
<% end %>

Answer 2

You are getting the error because #post.user is nil in :user_id => #post.user.id.
Make sure you define #post in your post controller's show action and that it has a valid user association.